In engines of passenger cars, motorcycles, agricultural machines, and the like, a crankshaft is required for taking out power by converting reciprocating motion of pistons to rotary motion. Generally, there are two types of crankshafts: those that are manufactured by forging and those that are manufactured by casting, and the former forged crankshafts superior in terms of strength and stiffness are more widely used. In recent years, in order to improve fuel economy performance and meet emission regulations, downsizing of engine displacement becomes popular, and a three-cylinder engine is attracting wide attention.
In general, forged crankshafts for three-cylinder engines are manufactured by using, as a starting material, a billet having a circular or square cross section and having a constant cross-sectional area along the entire length, and subjecting the billet to the steps of preforming, die forging, trimming and coining in order. The preforming step includes roll forming and bending (so called “preforming”), and the die forging step includes block forging and finish forging.
FIG. 1 is a schematic diagram illustrating a typical conventional process for manufacturing a forged crankshaft for a three-cylinder engine. A crankshaft 1 illustrated in FIG. 1 is to be mounted in a three-cylinder engine. It is a three-cylinder four-counterweight crankshaft that includes: four journals J1 to J4; three crank pins P1 to P3; a front part Fr; a flange Fl; and six crank arms (hereinafter referred to as “arms” to be simple) A1 to A6 that alternatively connect the journals J1 to J4 and the crank pins P1 to P3 to each other, wherein among the six arms A1 to A6, first and second arms A1 and A2, and fifth and sixth arms A5 and A6 respectively connecting to first and third crank pins P1 and P3 at opposite ends, have balance weights. Hereinafter, when the journals J1 to J4, the crank pins P1 to P3, and the arms A1 to A6 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the crank pins, and a reference character “A” for the arms.
According to the manufacturing method shown in FIG. 1, the forged crankshaft 1 is manufactured in the following manner. Firstly, a billet 2 shown in FIG. 1(a), which has been previously cut to a predetermined length, is heated by an induction heater or a gas atmosphere furnace and then is subjected to roll forming. In the roll forming step, the billet 2 is rolled and reduced in cross section by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 103, which is an intermediate material, is formed (see FIG. 1(b)). In the bending step, the rolled blank 103 obtained by the roll forming is partially pressed in a press in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 104, which is a secondary intermediate material, is formed (see FIG. 1(c)).
Then, in the block forging step, the bent blank 104 obtained by bending is press forged with a pair of upper and lower dies, whereby a forged blank 105 having a general shape of a crankshaft (forged final product) is formed (see FIG. 1(d)). Then, in the finish forging step, the block forged blank 105 obtained by the block forging is further processed by press forging the block forged blank 105 with a pair of upper and lower dies, whereby a forged blank 106 having a shape in agreement with the shape of the crankshaft is formed (see FIG. 1(e)). In the block forging and the finish forging, excess material flows out as a flash from between the parting surfaces of the dies that oppose each other. Thus, the block forged blank 105 and the finish forged blank 106 have large flashes 105a and 106a, respectively, around the formed shape of the crankshaft.
In the trimming step, the finish forged blank 106 with the flash 106a, obtained by the finish forging, is held by dies from above and below and the flash 106a is trimmed by a cutting die. In this manner, the forged crankshaft 1 is obtained as shown in FIG. 1(f). In the coining step, principal parts of the forged crankshaft 1, from which the flash has been removed, e.g., shaft parts such as the journals J, the crank pins P, the front part Fr, and the flange Fl, and in some cases the arms A, are slightly pressed with dies from above and below and formed into a desired size and shape. Finally, the forged crankshaft 1 is manufactured.
The manufacturing process shown in FIG. 1 is applicable not only to a three-cylinder four-counterweight crankshaft as exemplified, but also to a three-cylinder six-counterweight crankshaft in which, all six arms A have balance weights. It should be noted that, when adjustment of a placement angle of the crank pins is necessary, a step of twisting is added after the trimming step.
With such a manufacturing method, however, it is inevitable that material utilization decreases since large amounts of unnecessary flash, which is not a part of the end product, are generated. Thus, in the manufacturing of a forged crankshaft, it has been so far an important object to inhibit the generation of flash to the extent possible and achieve improvement of material utilization. Examples of conventional techniques that address this object are as follows.
For example, Patent Literature 1 discloses a technique for manufacturing a crankshaft, the technique including: using, as a blank, a stepped round bar having reduced diameter regions at portions to be formed into journals and crank pins of a crankshaft; holding, with dies, a pair of the portions to be formed into journals, between which a portion to be formed into a crank pin is disposed and, in this state, axially moving the opposing dies toward each other to compressively deform the round bar blank; pressing punches against the portion to be formed into a crank pin in a direction perpendicular to the axial direction to place the portion to be formed into a crank pin into an eccentric position; and repeating the above operations in succession for all crank throws, whereby the journals and the crank pins are shaped and the arms are roughly shaped.
Further, Patent Literature 2 discloses a technique for manufacturing a crankshaft, the technique including: using, as a blank, a simple round bar; holding one end of the two ends of the round bar with a stationary die and the other end thereof with a movable die, and holding portions to be formed into journals with journal dies and portions to be formed into crank pins with crank pin dies; in this state, axially moving the movable die, the journal dies, and the crank pin dies toward the stationary die to compressively deform the round bar blank; and moving the crank pin dies in an eccentric direction perpendicular to the axial direction to place the portion to be formed into the crank pin into an eccentric position, whereby the journals and the crank pins are shaped and the arms are roughly shaped.
With both the techniques disclosed in Patent Literatures 1 and 2, no flash will be generated, and therefore a significant improvement in material utilization can be expected.